1. Field of the Invention
This invention relates to an electroluminescent device with an excellent luminescent efficiency.
2. Description of the Related Art
An organic electroluminescent device (hereinafter we call xe2x80x9corganic EL devicexe2x80x9d) is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode.
After C. W. Tang et al. of Eastman Kodak Co., reported a low-voltage-driven organic electroluminescent device using a double layered structure. Tang et al. have used tris(8-quinolinol)-aluminum in a light-emitting layer and a triphenyl diamine derivatives in a hole-transporting layer. This stacked structure gives such advantages as an improvement in the injection efficiency of holes into the light-emitting layer; blocking of electrons injected from a cathode, which increase the efficiency of exciton production from charge recombination; and confinement of the excitons into the light-emitting layer. A double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and light-emitting layer or a triple layered structure composed of a hole-injecting and transporting layer, a light-emitting layer and an electron-injecting and transporting layer is well known as an organic EL device. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
An EL device, however, has a limitation for its luminescent efficiency, since the singlet generation ratio caused by carrier recombination has a dependence on spin statistics of carriers. In organic EL device, a light which is emitted with larger outgoing angle than critical angle is totally reflected caused by the refractive index of the light emitting layer and cannot go out from the layer, light emitting layer to be 1.6, it may utilize only about 20% of the total emission. Combining with generation ratio as above, the total energy conversion efficiency may be limited to a low value, up to about 5% (Tetsuo Tsutsui, xe2x80x9cCurrent conditions and trends in an organic electroluminescencexe2x80x9d, Display(monthly), Vol. 1(No. 3), p. 11, September 1995) . Because of strong limitation of luminescent efficiency in organic EL device, the poor optical coupling efficiency from the light emitting layer to outside result in fatally lowered total efficiency.
There have been various attempts to improve the efficiency of optical coupling from the light emitting layer to outside for a light emitting device which have an equivalent structure to an organic EL device such as an inorganic EL device. For example, there have been procedures such as improving the efficiency by forming or attaching light convergent optics on a substrate as described in JP-A 63-314795 and forming a reflecting surface, e.g., on the side face of a device as described in JP-A 1-220394. These procedures are effective for a device which have a larger emission area. However, in a device with a smaller pixel area for, e.g., a dot matrix display, it may be difficult to form a lens for light convergence or a reflecting surface on a side face. Furthermore, because EL device has a thin make tapered structure for forming a reflecting mirror on a side face of the device and result in that a layer with an intermediate refractive index is placed as an antireflection film between a glass substrate and luminescent layer in JP-A 62-172691. This procedure may, however, improve a forward optical coupling efficiency, but not prevent total reflection. Thus, it may be effective for an inorganic EL device which has a larger refractive index, but not very effective for an organic EL device whose emitting layer is a relatively lower refractive index.
As described above there have been no satisfactory optical coupling methods EL device. Thus, developing such a efficient optical coupling methods is essential for improving in a total efficiency of an organic EL device.
An objective of this invention is to improve a optical coupling efficiency in an organic EL device and to provide an organic EL device with a high efficiency.
To solve the problems, this invention provides an organic EL device having one or more than one organic thin layers one of the electrode has a concave shape to the luminescent layer. In this invention, the above problems can be solved by forming the structure that either the anode or the cathode has a concave shape to the luminescent layer. Such a structure may be provided by forming a plurality of small protrusions on the counter electrode.
For example, small protrusions formed on the surface of an electrode 2 as illustrated in FIG. 3 may allow readily providing a counter electrode 6 with a concave shape to a luminescent layer. The process will be described with referring to FIG. 1. An organic film 5 including a luminescent layer and a counter electrode 6 are sequentially deposited on an insulating layer 4 covering an electrode 2 on which a plurality of small protrusions 3 have been formed, resulting in concave parts on the organic film 5 and the counter electrode 6 corresponding to the small protrusions 3. Thus, a concave may be formed on the counter electrode 6 as a consequence of deposition.
As described above, the organic EL device of this invention has a concave electrode. A light generated in the organic film 5 including a luminescent layer is, therefore, reflected by the interface between the organic film 5 and the counter electrode 6, and then focused to the substrate 1 by a concave-mirror effect. Specifically, the light emitted from the area between these electrodes is reflected to a transverse direction to the substrate. Therefore, the light, which cannot be going out to the substrate would be coming out from substrate and the efficiency of optical coupling is remarkably improved. Since an emission area is relatively reduced and a forward reflection efficiency is not 100%, there is possibly to enhance luminance compared with a device having electrodes without small protrusions operating at the same voltage. Since reduction of an emission area may, however, reduce a power consumption, it leads to an overall high efficiency.
The small protrusions in the device of this invention are provided for endowing the counter electrode with a concave shape as described above. They may, therefore, have any shape as long as it provides such an effect; for example, a cylindrical, conical, truncated-cone, quadrangular-pyramid, truncated-pyramid, any cone, cone-pyramid, hemisphere or hemi-ellipsoid shape.
This invention also provides a process for manufacturing an organic EL device, comprising the steps of forming the first electrode layer on a substrate surface; forming a plurality of small protrusions on the first electrode layer; and sequentially forming one or more than one organic layers.
At first, after a flat electrode is formed as a base layer, on which an insulating layer with holes is then formed. Next is deposited into the holes to form small protrusions. In other cases, it may be possible to form a protrusion at each intersection in a grid patterned conductive electrode and the light outgoing from interstitial areas.
This invention further provides an organic EL device having one or more than one organic layers including a luminescent layer between an anode and a cathode, where either the anode or the cathode has inclined faces. An inclined face is defined as a surface inclined by a given angle from the plane of the substrate.
As illustrated in FIG. 8, a cathode 14 has inclined faces. Then, a light generated in a luminescent layer 13 is reflected by the interface between the luminescent layer 13 and the cathode 14, and then focused to the substrate 11 similar to the light emitted from the area between these electrodes which has a direction of totally reflected on the substrate surface or a parallel to the substrate surface may be reflected to a transverse direction to the substrate. Therefore, the light, which cannot be going out to the substrate would be coming out from substrate and the efficiency of optical coupling is remarkably improved since an emission area is relatively reduced and a forward reflection efficiency is not 100%, there is possibly to enhance compared with a device having electrodes without small protrusions operating at the same voltage. Since reduction an emission area may, however, reduce a power consumption, it lead to an overall high efficiency.
In this invention, it is preferable that a plurality of holes are formed on the counter electrode to the above electrode having inclined faces.
Thus, a counter electrode having inclined faces to a luminescent layer may be readily formed. The process will be described with referring to FIG. 7. A luminescent layer 13 and a counter electrode 14 are sequentially formed on an electrode 12 having a plurality of holes. Then, pits appear on the points on the luminescent layer 13 and the counter electrode 14 corresponding to the holes. In other words, the inclined faces may be formed as a consequence of deposition.
The holes in the device of this invention are provided for endowing the counter electrode with inclined faces as described above. They may, therefore, have any shape as long as it provides such an effect; for example, a stripe geometry such as a square, a rectangle and an oblong, and a circle.
These holes do not form isolation grooves. Specifically, the electrodes form a single pixel, and are not separated by the holes (FIG. 7). In other words, the electrode structure does not form a isolation groove between stripe electrodes like in the dot-matrix display forming horizontal and vertical pixel arrays, but provides small holes as internal structures in these electrodes.
This invention further provides a process for manufacturing an organic EL device, comprising the steps of forming the first electrode layer on a substrate; forming a plurality of holes on the first electrode layer; and sequentially forming one or more than one organic layers including a luminescent layer and the second electrode layer on organic layers. The first electrode layer is one of the pair electrodes positioned in the side of an emission face, while the second electrode layer is the counter electrode to the first electrode. For example, in FIG. 7, the anode 12 and the cathode 14 are the first and the second electrodes, respectively. According to the process, the counter electrode having inclined faces to the luminescent layer may be formed as a consequence of deposition.
A plurality of holes may be formed by, for example, the following procedure. A photo-resist is applied in a predetermined pattern on the surface of the first electrode. Then, a given part of the first electrode is etched off to form a plurality of holes. According to the procedure, a plurality of holes may be readily formed in a desired pattern.